Geochemistry and diagenetic history of the Ordovician Lower Head Formation sandstones, western Newfoundland, Canada

Abstract

The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises siltstones with very fine grained to fine-grained sandstones. Petrography confirms that these sandstones are matrix rich, essentially wackes, with detrital minerals including quartz, feldspar, biotite, and numerous accessory minerals. Observed petrographic features suggest that the Lower Head sediments are the distal product of erosion, with sediment sourced from the Dashwoods microcontinent and Lushs Bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin. The Lower Head Formation sandstones have low porosity, with early diagenetic cements (C1) and later calcite in crosscutting calcite veinlets (C2). Petrographic, isotopic, and fluid inclusion analyses indicate that C1 cements formed during the early stages of diagenesis. Both δ13C and δ18O isotopes for the C1 calcite cements are isotopically heavier than the C2 calcite veins. Fluid inclusion homogenization temperatures for the later crosscutting C2 calcite generally range between 78 and 116 °C, with a mean of 100.7 °C (±9.75 °C) and fluid salinities of 5.41–15.98 equiv. wt.% NaCl. Fluid inclusion gas analysis from C2 calcite confirms that CO2/CH4 generally has an inverse correlation with N2/Ar. Petroleum-bearing fluid inclusions were also recorded in C2 calcite cements, indicating that these fractures were conduits for hydrocarbon migration prior and (or) during cementation. However, the early cementation and associated low porosity of the Lower Head Formation sandstones indicate that they offer restricted pathways to migrating fluids and volatiles, and any hydrocarbon migration must have been fracture controlled.